fix text fields and exists operator

src/Schema/Ui/Slug.php

@@ -32,7 +32,7 @@ class Slug implements FieldInterface, RequiredInterface
 
     public function format(array $input, array $output): array
     {
-        $value = (string)$input[$this->name] ?? null;
+        $value = !empty($input[$this->name]) ? (string)$input[$this->name] : null;
         if(empty($value)){
             $value = Str::slug($input[$this->source]);
         }